Add CODEOWNERS for Actions workflows

Author: henrymercer

CODEOWNERS

@@ -31,3 +31,9 @@
 # Documentation etc
 /*.md @github/code-scanning-product
 /LICENSE @github/code-scanning-product
+
+# Workflows
+/.github/workflows/ @github/codeql-ci-reviewers
+/.github/workflows/js-ml-tests.yml @github/codeql-ml-powered-queries-reviewers
+/.github/workflows/ql-for-ql-* @github/codeql-ql-for-ql-reviewers
+/.github/workflows/ruby-* @github/codeql-ruby